Heretofore, in the continuous pressure treatment of moving webs, it has been known to use pairs of rollers to compress the web as the web passes through the nip formed by each roller pair. Thus, the zone of action of the pressure is only the narrow roller nip, which in theory is merely a line but in practice, due to the thickness and compressibility of the material and the deformation of the roller surface, is a narrow zone. To enable treatment to be performed adequately, therefore, the treatment must often be repeated. Thus, the web has been guided through a number of successive roller nips, one after the other. This is done in known manner, for instance, in a paper calender which normally comprises stacks of up to twelve superimposed rollers over which the paper web is guided, being looped alternately over the successive rollers. The required pressure treatment is performed between each pair of rollers, and the total operative surface is the sum of the narrow pressure zones of the individual roller nips. However, this involves a considerable expense due to the large number of rollers needed to obtain the operative surface, a large roller stand and many drives.
In order to overcome the disadvantages of these systems, it has been known to use an apparatus for the continuous pressure treatment of webs in which the pressure is operative over a larger area than with rollers, without the expense of providing a large number of rollers connected one after the other. The basic concept of this apparatus is to extend the working nip, in which pressure is exerted between two rollers, over the surface of one roller or drum thus increasing the operative surface. To this end, the operative surface is increased by substituting a concurrently movable forming belt for the matching roller and by partly looping the forming belt around the drum. The web is guided between the co-rotating forming belt and drum and is pressure treated therebetween.
In this latter apparatus, the pressure which can be exerted on a web corresponds to the pressure under which the forming belt bears against the drum, and is determined by the tensioning of the forming belt. Such tensioning is usually subjected to fairly precisely defined limits. The forming belt is, of course, endless and must be deflected several times over rollers or drums in its path. The resulting bending causes tensile and compressive stresses in the material of the forming belt which must remain in the resilient zone, i.e., below the flow limit of the material of the belt. Since it is not possible to increase the drum radius a sufficient amount to improve the bending geometry, there is a maximum permissible belt thickness for any particular construction. From this belt thickness the maximum permissible tensile loading of the belt is determined. The resulting radial pressures are, however, inadequate to produce on a web, for instance, an effect comparable with the work of a calender.
German Pat. No. 923,172 issued in July 1955 discloses an apparatus for continuous pressing which includes a rotating drum having a steel forming belt looped around the surface by about 120.degree., the material to be pressed being disposed between the forming belt and the drum. In addition, a segment which has a supporting surface adapted to the outer shape of the forming belt is disposed outside of the forming belt and is pressed radially against the drum. Also, rolling members which transmit pressure from the supporting surface to the forming belt are rollably disposed between the steel belt and the supporting surface. When pressure is exerted in this way, there is no need for special belt tensioning, which is difficult to control.
However, the radial application of pressure by the supporting surface causes radial forces on the drum which must be absorbed by the drum bearing. Consequently, in the case of an apparatus for processing fairly wide webs such as, for instance, webs of paper, and also, if the zone of application of pressure is to be widened in the peripheral direction, bearing loadings can cause considerable technical problems. The expense of the apparatus would also be increased if attempts are made to attain pressures comparable with arrangements of rollers.
One solution to some of these problems is disclosed in U.S. Pat. No. 3,799,052 granted Mar. 26, 1974. This U.S. Pat. No. 3,799,052 discloses an apparatus for the continuous pressure treatment of a web comprising a rotatable drum, a movable forming belt looped about the drum and a sleeve forming a supporting surface about the belt. The sleeve is disposed about the drum surface for substantially more than 180.degree. with a pair of opposed ends in spaced apart relation to form an axial nip for passage of the belt. In addition, friction-reducing means are provided between the sleeve and belt and means are interconnected to the ends of the sleeve to impart a tensile stress in the sleeve.
The sleeve is looped around the drum by more than 180.degree. to terminate in a zone in which the ends of the sleeve approach one another after being looped around the drum. As a result of this and the tensile force existing between the ends of the sleeve, the sleeve can act like a tensioning belt whose tensioning in the peripheral direction exerts a radial force on the forming belt and the drum. The force corresponds to the pressure exerted on the web. Whereas in the prior art arrangement the force was directed from the supporting surface against the drum from one side, now, at least, a large proportion of the force is left inside the sleeve and kept away from the drum bearing.
As is more fully disclosed in that U.S. Pat. No. 3,799,052, which is hereby incorporated by reference, the tensioning arrangement results in a reduced loading of the support bearings of the rotating apparatus. Although that apparatus works quite well, the bearing still is loaded to some degree and thus, there is a need for an improved arrangement of this nature which offers the advantages of the aforesaid arrangement, while at the same time, further reducing or eliminating bearing forces.